The present invention relates in general to a method and an apparatus for controlling an electromagnetic clutch for use on an automotive vehicle, and more particularly to an improved technique by which the electromagnetic clutch is controlled so as to absorb a variation in output torque of the engine for improved drivability of the vehicle, yet with a minimum power loss in the electromagnetic clutch for economical operation of the engine.
For improved fuel economy of a vehicle engine, it has been known in the art to operate the engine at a relatively low speed with a relatively high torque by controlling or changing a speed ratio of the transmission. When the engine is operated in such condition, i.e., in a low-speed high-torque mode, the ignition period or cycle time of the engine tends to be long and consequently the engine suffers increased periodic variations in its output torque, whereby the drivability of the vehicle is degraded due to vibrations and noises which are caused by the periodic variations of the engine torque. In such arrangement, the average output torque of the engine is sufficient to drive the vehicle. However, there remains a problem of drivability when the engine is run at speeds in a relatively low range.
In the meantime, an electromagnetic clutch using a mass of magnetic powder was developed, as disclosed in Japanese Patent Application which was laid open in 1983 under Publication No. 58-657. Such an electromagnetic clutch is disposed between a vehicle engine and a transmission to transmit an output of the engine to the transmission. The clutch is controlled so that a rotating speed of its output shaft whose torque is transmitted to the transmission is lower than a rotating speed of its input shaft by a predetermined amount which is greater than a magnitude of variation in the rotating speed of the input shaft (variation from the average speed) which is caused by periodic variations of the torque of the input shaft. Namely, a slight amount of slip which gives a speed differential between the input and output shafts, is positively given to the electromagnetic clutch, so that a variation in output torque of the engine may be absorbed or accommodated by the clutch, in order to permit the engine to be operated in a low-speed high-torque mode with a high fuel economy, while maintaining a high level of drivability of the vehicle.
In the above-described arrangement for controlling the electromagnetic clutch, however, it is difficult for various reasons, to achieve an intricate control of an amount of slippage between the input and output shafts of the clutch, for absorbing the torque variation of the engine while minimizing the transmission power loss within the clutch.
Stated in greater detail, the magnitude of the engine torque variation varies depending upon current levels of speed and torque, and other specific operating conditions of the engine. Consequently, the control to maintain a constant amount of slip of the electromagnetic clutch (constant speed difference between its input and output shafts) does not make it possible to simultaneously accomplish the minimization of the power loss and the absorption of the engine torque variation, in the engine range of the varying operation conditions of the engine. With a constant amount of slip, the clutch slips too much and the fuel economy of the engine is lowered when the engine is operated with a relatively small level of variation in its output torque. If the amount of slip is reduced, on the contrary, the clutch is not able to sufficiently absorb the engine torque variation and the drivability of the vehicle is lowered when the engine is operated with a relatively large torque.
To solve the above inconveniences, it is considered to control the transmission torque of the electromagnetic clutch so that the amount of slip of the clutch is varied based on the detected output conditions of the engine. In this case, the amount of slip of the clutch may be optimized to the varying magnitude of variation in the engine torque. However, this method is available only where the output characteristics of the engine are unchanged. That is, it is impossible to give the clutch an optimum amount of slip that meets the varying magnitude of the engine torque, when the clutch is used for different types of engines or when the same engine is subject to changes in operating conditions such as atmospheric pressure, temperature of cooling water, relative humidity of the atmosphere, amount of carbon deposit in combustion chambers, etc., or changes in output characteristics.